This invention relates to methods of producing molded precision polyurethane products, especially charging rollers. It further relates to apparatuses for producing such products
Common processes for thermosetting polyurethane include casting and reaction injection molding (RIM). However, there are some limitations to both of these processes.
Generally, in casting processes, each component of the resin is mixed in a dynamic rotary pin or propeller mixer, and the mixed material is then poured into an open mold. Sometimes, the mold is sealed with an upper side mold piece after the mixed material is poured into the bottom side mold piece. Because the process is done in an open air atmosphere, it is difficult to eliminate air bubbles from the molded parts.
Casting processes are primarily used in the industry for thermosetting polyurethane rollers for paper handling, or for electric and static controlling rollers. Generally, a quadrilateral mold or a U-shaped mold is used for casting a cylindrical part with a shaft at its center. The shapes of these molds lead to a big loss of material and additional grinding processes. Additionally, it is very difficult to prevent air bubbles from forming in the molded parts. This is because after the material is mixed, the material is handled in an open atmosphere environment prior to pouring it into an open mold.
Furthermore, it is very important, especially for a highly functional molded part such as a charging roller for a photo conductor of an electrophotographic printing system, for the final mixture to have uniform properties. However, it is difficult to make uniformly molded parts using a quadrilateral mold or a U-shaped mold, because it is difficult to keep the mixture at a uniform temperature both because it is difficult to apply heat to the mixture uniformly and because thermosetting polyurethane generates heat. These problems lead to different heat histories encapsulated in the same molded part.
RIM processes are very good processes for producing specially designed foam-molded parts. RIM processes generally involve mixing components at high pressures. Components are injected at very high pressures into a very tough mold that is holed in a press. The mold is kept highly pressurized, so that the curing time is kept short, and process efficiency is increased. This is a big limitation of RIM processes. It is difficult to make a softer thermosetting polyurethane resin using a RIM process because of the generally longer curing time.
It is possible to apply RIM processes to mold solid thermosetting polyurethane, but it is difficult to avoid getting air bubbles in the molded parts due to the high pressure mixing system.
It is also evident from the fact that almost nobody is using RIM processes to make rollers that RIM processes are not good processes for this industry.
In the liquid silicone industry, some people use a third system to make molded parts. Each component is injected into a static mixer using syringe type metering pumps, and the mixture is injected into a mold directly from the outlet side of the mixer. This is sometimes referred to as liquid injection molding (LIM). This system is possible in liquid silicone systems because of liquid silicone""s profile of reaction, and its particular component system. Generally, silicone suppliers supply liquid silicone systems as one or two component systems. For molded parts, generally two component systems are popular. The flow rate and the viscosity of each component are generally equal. The reason this is possible in the silicone industry is that oil or other additives may be added to adjust the rate of flow and viscosity. However, additives cannot be used to make most thermosetting polyurethane products especially for high performance parts. In order to avoid inferior quality polyurethane products, we need to reduce the presence of components that cannot chemically react.
Additionally, we do not need to heat up the components in a liquid silicone system, so it is easy to control the reaction speed.
However, this LIM system does not work well for thermosetting polyurethane, except in very limited systems, such as where we have a two component system, where the two components are mixed in roughly the same proportions, where the viscosity of the two components is almost the same, and where the chemical reaction speed is very slow at room temperature. Most compositions of thermosetting polyurethane products do not possess such ideal properties.
The primary objective of the invention was to devise a method and apparatus for making molded thermosetting polyurethane parts at a lower cost, with lower material loss, and with a lower scrap rate. Subordinate to this main goal, was the secondary goal of increasing the homogeneity of a mixture containing materials that varied widely in amounts and viscosity.
In order to create a cost-effective and high-performance process of making molded thermosetting polyurethane parts and rollers, we have developed a complete molding system and a process for using it. It uses some concepts that already exist in the prior art, but combines them in a new manner. Two components of the mixing system, the premixing chamber and the dual-path mixer, are also claimed as separate inventions.
The invention is a method and apparatus for mixing and injection molding thermosetting polyurethane. The invention involves using a premixer that comprises a series of specially designed premixing chambers, one or more static, rather than dynamic, mixers, and a specially designed dual-path mixer. After the material is mixed it is pumped into an injector, and then deposited into a mold.
This entire process is accomplished in an airtight environment. Valve systems are in place both before the premixer and after the dual-path mixer. The injector is hermetically attached to an inlet of the mold. The mold into which the material is pumped is attached to a vacuum pump that decreases the chances of any air bubbles being trapped inside the mold.
This invention also increases the quality of injection molded parts by increasing the homogeneity of the mixture prior to molding. Doing this ensures that the end products of production will be uniform throughout. This is accomplished in part by utilizing a new dual-path mixer. A new type of premixing chamber further increases the blending of the individual components.
Another, more specific objective of this invention was to devise a method and apparatus for making functional thermosetting polyurethane rollers with highly uniform properties. This method and apparatus generates high quality polyurethane rollers.
While this system will work for many different compositions of thermosetting polyurethane, it handles the following situations especially well:
(1) molding systems where more than two components make up the final material;
(2) molding systems where the components are present in differing amounts, such as where the ratio of two components is 2:1 or greater;
(3) molding systems where the viscosity of the mixture is over 2000 cps at the molding temperature;
(4) molding systems where there is a big difference in viscosity of components;
(5) molding systems where the pot-life of mixture at the molding temperature is between 3 minutes and 60 minutes.